The innate immune response is dependent upon a discrete collection of proteins that mediate the detection of microbial invaders and their elimination. Only a handful of these proteins are known at present, and among them, the Toll-like receptors and associated signaling proteins play a prominent role as sensors. The program of which this Bridging proposal is a part will permit the identification of many critical genes involved in the innate immune response. We propose to examine each of these genes in turn, starting with the Toll-like receptors and moving to new genes as they are discovered. Our ultimate goal is to determine whether mutations at these loci contribute to susceptibility to sepsis in humans, or influence the outcome of sepsis once it is established. The approach taken is one in which modem techniques for mutation detection will be used to assay genetic load within the coding region at each locus, in disease populations and in ethnically matched control populations. In particular, we will emphasize the use of advanced methods for mutation detection, coupled with high-throughput sequencing to achieve this end. In accordance with the emerging principle that most complex human diseases may be ascribed to low-frequency codominant mutations affecting multiple loci, it is expected that mutations will be over-represented in specific loci within a disease population, given that those loci encode genes that are important in fighting infection. Preliminary work has supported this principle as it applies to the TLR4 locus in meningococcal disease: there is a significant excess ofmissense mutations at this locus in the disease population, compared with controls. We intend to generalize the principle, seeking to establish the importance of rare mutations at multiple loci in the pathogenesis of diverse infections. This proposal will entail extensive interactions between the Forward Genetics Core, the Genomics Core, and the Sequencing Core. Mutational data bearing on the involvement of specific genes in sepsis will quickly be disseminated to the scientific community at large through a webbased information system. Ultimately, these studies may foretell susceptibility to infection, and explain the strong heritability of infectious diseases in molecular terms.